Electro-optic devices are critical components of optical communication systems. By electrically changing the refractive index of material in an optical pathway, they can switch, attenuate or modulate an optical signal.
A commonly used electro-optic device, e.g. an optical modulator, uses an electrical field to control the behaviour of light in a waveguide path at the surface of an electro-optic substrate (e.g. a lithium niobate (LiNbO3) substrate). The electrical field applied to the waveguide region can vary the refractive index in the path. The electrode used to apply the field is generally a conventional metallic electrode placed directly on the substrate. However, in order to prevent high optical loss, the metallic electrode has to be placed at some distance from the waveguide. This results in low electro-optic efficiency. This is defined as high Vπ.
Alternatively, a buffer layer could be provided between the waveguide and the metal electrode. This in turn offers improved Vπ of the modulator. Materials such as SiO2 (which is optically transparent) are quite commonly used as the buffer layer. Nevertheless the buffer layers suffer from strong DC drift effects, which are potentially dangerous for the long-term stability of the device. This needs to be controlled and stabilized at a given operating point (e.g., quadrature, maximum, minimum) to achieve good performance. Another known solution is a buffer made entirely of transparent conductor, e.g. indium tin oxide (ITO).
Such a transparent conductor is a semiconductor material which possesses both electrical conductivity and low optical losses (e.g., ITO or other similar materials). Conventionally, in this way, such electrodes have a three-layer structure so that the transparent buffer layer is formed on the optical waveguide with a metal electrode on top of it.
Suitable arrangements for such a three-layer structure are disclosed in U.S. Pat. No. 6,853,757 and U.S. Pat. No. 6,480,633. For the arrangements of these documents, it is possible that light leaks out from the transparent electrode to the metal electrode, and in particular, under high-speed modulation, optical loss caused by leaking out cannot be disregarded.
Thus there is a need for an improved bias electrode design for the electro-optic devices, which is capable of reducing optical loss.